With the advent of modern antipollution laws in the United States and around the world, significant and new methods of minimizing various pollutants are being investigated. The burning of fuel, be the fuel wood, coal, oils, or natural gas, likely causes a majority of the pollution problems in existence today. Certain pollutants such as SO.sub.2, which are created as the result of the presence of a contaminant in the fuel source, may be removed either by treating the fuel to remove the contaminant or by treating the exhaust gas eventually produced to remove the resulting pollutant. Pollutants such as carbon monoxide, which are created as the result of incomplete combustion, may be removed by post-combustion oxidation or by improving the combustion process. The other principal pollutant, NO.sub.x (an equilibrium mixture mostly of NO, but also containing very minor amounts of NO.sub.2), may be dealt with either by controlling the combustion process to minimize its production or by later removal. Removal of NO.sub.x once produced once it is a difficult task because of its relative stability and its low concentration in most exhaust gases. One ingenious solution used in automobiles is the use of carbon monoxide chemically to reduce NO.sub.x to nitrogen while oxidizing the carbon monoxide to carbon dioxide. However, the need to react two pollutants alsospeaks to a conclusion that the initial combustion reaction was inefficient.
It must be observed that unlike the situation with sulfur pollutants where the sulfur contaminant may be removed from the fuel, removal of nitrogen from the air fed to the combustion process is clearly an impractical solution. Unlike the situation with carbon monoxide, improvement of the combustion reaction would likely increase the level of NO.sub.x produced due to the higher temperatures then involved.
Nevertheless, the challenge to reduce combustion NO.sub.x remains and several different methods have been suggested. The process chosen must not substantially conflict with the goal for which the combustion gas was created, i.e., the recovery of its heat value in a turbine, boiler, or furnace.
Many recognize that a fruitful way of controlling NO.sub.x production is to limit the localized and bulk temperatures in the combustion zone to something less than 1800.degree. C. See, for instance, U.S. Pat. No. 4,731,989 to Furuya et al. at column 1, lines 52-59 and U.S. Pat. No. 4,088,435 to Hindin et al. at column 12.
There are a number of ways to control the temperature, such as by dilution with excess air, controlled oxidation using one or more catalysts, or staged combustion using variously lean or rich fuel mixtures. Combinations of these methods are also known.
One widely attempted method is the use of multistage catalytic combustors. Most of these processes utilize multi-section catalysts with metal oxide or ceramic catalyst carriers. Typical of such disclosures are:
__________________________________________________________________________ Country Document 1st Stage 2nd Stage 3rd __________________________________________________________________________ Stage Japan Kokai 60-205129 Pt-group/Al.sub.2 O.sub.3 & SiO.sub.2 La/SiO.sub.2.Al.sub.2 O.sub.3 Japan Kokai 60-147243 La & Pd & Pt/Al.sub.2 O.sub.3 ferrite/Al.sub.2 O.sub.3 Japan Kokai 60-66022 Pd & Pt/ZrO.sub.2 Ni/ZrO.sub.2 Japan Kokai 60-60424 Pd/- CaO & Al.sub.2 O.sub.3 & NiO & w/noble metal Japan Kokai 60-51545 Pd/* Pt/* LaCoO.sub.3 /* Japan Kokai 60-51543 Pd/* Pt/* Japan Kokai 60-51544 Pd/* Pt/* base metal oxide/* Japan Kokai 60-54736 Pd/* Pt or Pt--Rh or Ni base metal oxide or LaCO.sub.3 /* Japan Kokai 60-202235 MoO.sub.4 /- CoO.sub.3 & ZrO.sub.2 & noble metal Japan Kokai 60-200021 Pd & Al.sub.2 O.sub.3 /+* Pd & Al.sub.2 O.sub.3 /** Pt/** Japan Kokai 60-147243 noble metal/heat resistant carrier ferrite/heat resistant carrier Japan Kokai 60-60424 La or Nd/Al.sub.2 O.sub.3 0.5% SiO.sub.2 Pd or Pt/NiO & Al.sub.2 O.sub.3 & CaO 0.5% SiO Japan Kokai 60-14938 Pd/? Pt/? Japan Kokai 60-14939 Pd & Pt/refractory ? ? Japan Kokai 60-252409 Pd & Pt/*** Pd & Ni/*** Pd & Pt/*** Japan Kokai 60-080419 Pd & Pt Pd, Pt & NiO Pt or Pt & Pd Japan Kokai 60-080420 Pd & Pt & NiO Pt Pt & Pd Japan Kokai 60-080848 Pt & Pd Pd & Pt & NiO Pt or Pt & Pd Japan Kokai 60-080849 Pd, Pt, NiO/? Pd & Pt (or NiO)/? Pt or Pd & __________________________________________________________________________ Pt/? *alumina or zirconia on mullite or cordierite **Ce in first layer; one or more of Zr, Sr, Ba in second layer; at least one of La and Nd in third layer. ***monolithic support stabilized with lanthanide or alkaline earth metal oxide Note: the catalysts in this Table are characterized as "a"/"b" where "a" is the active metal and "b" is the carrier
The use of such ceramic or metal oxide supports is clearly well-known. The structures formed do not readily melt or oxidize as would a metallic support. A ceramic support carefully designed for use in a particular temperature range can provide adequate service in that temperature range. Nevertheless, many such materials can undergo phase changes or react with other components of the catalyst system at temperatures above 1100.degree. C., e.g., the gamma alumina phase changes to the alpha alumina form in that region. In addition, such ceramic substrates are olefin fragile, subject to cracking and failure as a result of vibration, mechanical shock, or thermal shock. Thermal shock is a particular problem in catalytic combustors used in gas turbines. During startup and shutdown, large temperature gradients can develop in the catalyst leading to high mechanical stresses that result in cracking and fracture.
Typical of the efforts to improve the high temperature stability of the metal oxide or ceramic catalyst supports are the inclusion of an alkaline earth metal or lanthanide or additional metals into the support, often in combination with other physical treatment steps:
______________________________________ Country Document Assignee or Inventor ______________________________________ Japan Kokai 61-209044 (Babcock-Hitachi KK) Japan Kokai 61-216734 (Babcock-Hitachi KK) Japan Kokai 62-071535 (Babcock-Hitachi KK) Japan Kokai 62-001454 (Babcock-Hitachi KK) Japan Kokai 62-045343 (Babcock-Hitachi KK) Japan Kokai 62-289237 (Babcock-Hitachi KK) Japan Kokai 62-221445 (Babcock-Hitachi KK) U.S. U.S. Pat. No. 4,793,797 (Kato et al.) U.S. U.S. Pat. No. 4,220,559 (Pliknki et al.) U.S. U.S. Pat. No. 3,870,455 (Hindin et al.) U.S. U.S. Pat. No. 4,711,872 (Kato et al.) Great Britain 1,528,455 Cairns et al. ______________________________________
However, even with the inclusion of such high temperature stability improvements, ceramics are fragile materials. Japanese Kokai 60-053724 teaches the use of a ceramic columnar catalyst with holes in the column walls to promote equal distribution of fuel gas and temperature amongst the columns lest cracks appear.
High temperatures (above 1100.degree. C.) are also detrimental to the catalytic layer resulting in surface area loss, vaporization of metal catalysts, and reaction of catalytic components with the ceramic catalyst components to form less active or inactive substances.
Of the numerous catalysts disclosed in the combustion literature may be found the platinum group metals: platinum, palladium, ruthenium, iridium, and rhodium; sometimes alone, sometimes in mixtures with other members of the group, sometimes with non-platinum group promoters or co-catalysts.
In addition to the strictly catalytic combustion processes, certain processes use a final step in which remaining combustibles are homogeneously oxidized prior to recovering the heat from the gas.
A number of the three stage catalyst combination systems discussed above also have post-combustion steps. For instance, a series of Japanese Kokai assigned to Nippon Shokubai Kagaku ("NSK") (62-080419, 62-080420, 63-080847, 63-080848, and 63-080549) disclose three stages of catalytic combustion followed by a secondary combustion step. As was noted above, the catalysts used in these processes are quite different from the catalysts used in the inventive process. Additionally, these Kokai suggest that in the use of a post-combustion step, the resulting gas temperature is said to reach only "750.degree. C. to 1100.degree. C.". In clear contrast, the inventive process when using the post catalyst homogeneous combustion step may be seen to reach substantially higher adiabatic combustion temperatures.
Other combustion catalyst/post-catalyst homogeneous combustion processes are known. European Patent Application 0,198,948 (also issued to NSK) shows a two or three stage catalytic process followed by a post-combustion step. The temperature of the post-combusted gas was said to reach 1300.degree. C. with an outlet temperature from the catalyst (approximately the bulk gas phase temperature) of 900.degree. C. The catalyst structures disclosed in the NSK Kokai are not, however, protected from the deleterious effects of the combustion taking place within the catalytic zones and consequently the supports will deteriorate.
The patent to Furuya et al. (U.S. Pat. No. 4,731,989) discloses a single stage catalyst with injection of additional fuel followed by post-catalyst combustion. In this case, the low fuel/air ratio mixture feed to the catalyst limits the catalyst substrate temperature to 900.degree. C. or 1000.degree. C. To obtain higher gas temperatures required for certain processes such as gas turbines, additional fuel is injected after the catalyst and this fuel is burned homogeneously in the post catalyst region. This process is complicated and requires additional fuel injection devices in the hot gas stream exiting the catalyst. The inventive device described in our invention does not require fuel injection after the catalyst; all of the fuel is added at the catalyst inlet.
An aspect in the practice of our inventive process is the use of integral heat exchange structures--preferably metal and in at least in the latter catalytic stage or stages of the oxidation. Generically, the concept is to position a catalyst layer on one surface of a wall in the catalytic structure which is opposite a surface having no catalyst. Both sides are in contact with the flowing fuel-gas mixture. On one side reactive heat is produced; on the other side that reactive heat is transferred to the flowing gas.
Structures having an integral heat exchange feature are shown in Japanese Kokai 59-136,140 and 61-259,013. Similarly, U.S. Pat. No. 4,870,824 to Young et al. shows a single stage catalytic combustor unit using a monolithic catalyst with catalysts on selected passage walls. In addition to a number of other differences, the structures are disclosed to be used in isolation and not in conjunction with other catalyst stages. Additionally, the staged use of the structure with different catalytic metals is not shown in the publications.
None of the processes shown in this discussion show a combination catalyst system in which the catalyst supports are metallic, in which the catalysts are specifically varied to utilize their particular benefits, in which integral heat exchange is selectively applied to control catalyst substrate temperature, and particularly, in which high gas temperatures are achieved while maintaining low NO.sub.x production and low catalyst (and support) temperatures.